Ink-jet printing is a non-impact printing process in which droplets of ink are deposited on print media, such as paper, transparency film, or textiles. Low cost and high quality of the output, combined with relatively noise-free operation, have made ink-jet printers a popular alternative to other types of printers used with computers. Essentially, ink-jet printing involves the ejection of fine droplets of ink onto print media in response to electrical signals generated by a microprocessor.
There are two basic means currently available for achieving ink droplet ejection in ink-jet printing: thermally and piezoelectrically. In thermal ink-jet printing, the energy for drop ejection is generated by electrically-heated resistor elements, which heat up rapidly in response to electrical signals from a microprocessor to create a vapor bubble, resulting in the expulsion of ink through nozzles associated with the resistor elements. In piezoelectric ink-jet printing, the ink droplets are ejected due to the vibrations of piezoelectric crystals, again, in response to electrical signals generated by the microprocessor. The ejection of ink droplets in a particular order forms alphanumeric characters, area fills, and other patterns on the print medium.
Inks useful in ink-jet printing are typically composed of either pigments or dyes. Pigments are very small insoluble solid colorant particles wherein the molecules are strongly associated with one another through intermolecular forces such as Van der Waals attraction, pi-pi interactions or hydrogen bonding. These forces of attraction prevent solvation of the molecules by common solvents such that they cannot form solutions like their dye counterparts. Conventionally, pigments are used in printing inks by suspending them in a liquid medium using a high-energy dispersion process with the aid of dispersing agents. A disadvantage of using pigments is that their dispersions are inherently thermodynamically unstable and eventually the pigment particles agglomerate to larger particles that tend to settle. Moreover, pigments lack uniform size distribution. Agglomerated and/or non-uniform sized particles lead to "bleeding" of the inks into one another when printed in various colors on paper substrates ink. In addition to bleed, the use of these pigments lead to longer drying time of the ink on the print media and to problems with pen reliability due to clogging and crusting of the ink in the pen mechanism.
Technology has been developed to encapsulate pigments to reduce agglomerization and provide more uniform size distribution. Surface modifications not involving encapsulating that render the pigment surface appropriate for suspension in solvents are also known. Other proposed solutions to the problem of pigment agglomeration are disclosed U.S. Pat. No. 5,531,816, entitled "Bleed-Alleviated, Waterfast, Pigment-Based Ink-Jet Ink Compositions," and U.S. Pat. No. 5,713,989 entitled "Bleed Alleviated Aqueous Pigment Dispersion-Based Ink-Jet Ink Compositions," both assigned to the same assignee as the present application. These disclosures are directed to bleed control of solvent-dispersed pigment-based and aqueous pigment-based ink-jet ink compositions, respectively. More specifically, bleed control for such inks is achieved by dispersing the deagglomerated pigment in solution by means of a microemulsion.
Although these solutions are promising, a need still exists for pigment-based inks for use in ink-jet printing that evidence reduced agglomeration, and thereby, reduced bleed, rapid drying upon impact with the print medium, and enhanced reliability in performance. Moreover, the print quality of the ink composition must not be sacrificed in order to achieve these performance benefits.
Recent literature has described a method for modifying insoluble pigments such that they are made soluble in organic solvents by introducing substituents that disrupt Van der Waals and pi-pi interactions and/or remove the ability of the pigment molecules to form intermolecular hydrogen bonds. These chemically modified pigments can then be converted back to their native form by the application of heat or radiation. These pigment precursors have been termed "latent" pigments.